xfstests: add a helper to get the minimum dio size

Various tests opencode checks to find out the minimum support direct I/O
size.  Replace those with a generic helper that handles network filesystems as
well.  Also remove the Linux 2.4 workaround we had in once place.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>

common/rc

@@ -2179,6 +2179,21 @@ _scale_fsstress_args()
     echo $args
 }
 
+#
+# Return the logical block size if running on a block device,
+# else substitute the page size.
+#
+_min_dio_alignment()
+{
+    dev=$1
+
+    if [ -b "$dev" ]; then
+        blockdev --getss $dev
+    else
+        $here/src/feature -s
+    fi
+}
+
 run_check()
 {
 	echo "# $@" >> $seqres.full 2>&1